Systems and methods for forming a time-averaged line image

ABSTRACT

Systems and methods for forming a time-average line image are disclosed. The method includes forming a line image with a first amount of intensity non-uniformity. The method also includes forming and scanning a secondary image over at least a portion of the line image to form a time-averaged modified line image having a second amount of intensity non-uniformity that is less than the first amount. Wafer emissivity is measured in real time to control the intensity of the secondary image. Temperature is also measured in real time based on the wafer emissivity and reflectivity of the secondary image, and can be used to control the intensity of the secondary image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 12/925,517, filed on Oct. 22, 2010, now U.S. Pat. No. 8,026,519, which application is incorporated by reference herein.

FIELD

The present disclosure relates generally to the use of line images, and in particular relates to systems and methods for forming a time-averaged line image having a relatively high degree of intensity uniformity.

BACKGROUND ART

There are a variety of applications that require the use of a line image having a relatively uniform intensity. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA), or just “laser annealing.” Laser annealing is used in semiconductor manufacturing for a variety of applications, including for activating dopants in select regions of devices (structures) formed in a semiconductor wafer when forming active microcircuits such as transistors.

One form of laser annealing uses a scanned line image from a light beam to heat the surface of the wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants in the semiconductor structures (e.g., source and drain regions) but short enough to prevent substantial dopant diffusion. The time that the wafer surface is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).

To achieve high wafer throughput in a commercial laser annealing system, the line image should be as long as possible, while also having a high power density. An example range for usable line-image dimensions is 5 mm to 100 mm in length (cross-scan direction) and 25 microns to 500 microns in width (scan direction). To achieve uniform annealing, it is also necessary for the intensity profile along the line-image length to be as uniform as possible, while non-uniformities along the line-image width are averaged out during the scanning process.

Typical semiconductor processing requirements call for the anneal temperature to be between 1000° C. and 1300° C., with a temperature uniformity of +/−3° C. To achieve this degree of temperature uniformity, the line image formed by the annealing light beam needs to have a relatively uniform intensity in the cross-scan direction, which under most conditions is less than a +/−5% intensity variation.

A CO₂ laser is a preferred light source for laser annealing applications because its wavelength (nominally 10.6 microns) is much longer than the size of most device features on the wafer. This is important because using a wavelength on the order of the size of the device features can lead to pattern-related variations in exposure. Thus, when the wafer is irradiated with the 10.6 micron wavelength light, the light scattering from the features is minimal, resulting in a more uniform exposure. In addition, a CO₂ laser emits a relatively high-intensity beam. However, the coherence length for a CO₂ laser is relatively long, typically several meters. This makes it unfeasible to use a binary optic approach to produce a line image with the required degree of intensity uniformity, i.e., ˜10% (i.e., about +/−5%) based on principles of Kohler illumination.

SUMMARY

The disclosure is generally directed to systems and methods for forming a time-average line image in a manner that substantially maintains process temperature control at the process location for a laser-based thermal annealing process. The systems and methods substantially reduce or eliminate systematic and stochastic non-uniformities in the thermal profile of the line image formed at the wafer surface by at least partially compensating for temperature metrology for emissivity variations of the wafer surface during laser annealing.

An aspect of the disclosure is a line-image-forming optical system for thermally annealing a semiconductor wafer having a surface. The system includes a primary optical system configured to form a line image at the wafer surface, the line image having long axis and a first amount of intensity non-uniformity along the long axis. The system also includes a secondary laser system that generates a secondary laser beam having a secondary laser beam wavelength and a secondary laser beam intensity. The system further includes a scanning optical system having a first field of view and configured to receive the secondary laser beam and form therefrom a secondary image at the wafer surface. The secondary image at least partially overlaps the line image and is scanned over at least a portion of the line image to form a time-averaged modified line image having a second amount of intensity non-uniformity that is less than the first amount. The system includes a thermal emission detection system configured to detect thermal emission light from the wafer surface through the scanning optical system and over a second field of view substantially the same as the first field of view of the scanning optical system. The thermal emission detection system is configured to generate an electrical signal corresponding to the detected thermal emission light. The system also has a controller configured to receive the electrical signal from the thermal emission detection system and in response thereto adjust at least one of the secondary laser beam intensity and a scanning speed of the secondary laser beam.

Another aspect of the disclosure is a method of forming a time-averaged modified line image at a surface of a semiconductor wafer. The method includes forming at the image plane a line image having a first amount of intensity non-uniformity in a long-axis direction. The method also includes forming and scanning a secondary image in the long axis direction over at least a portion of the line image while measuring an emissivity from a portion of the wafer surface associated with the scanned secondary image. The method further includes adjusting at least one of a scan speed and an intensity of the secondary image based on the measured emissivity to form a modified line image having a second amount of intensity non-uniformity in the long-axis direction that is less than the first amount.

Another aspect of the disclosure is a method of forming a time-averaged modified line image at a surface of a semiconductor wafer. The method includes forming at the image plane a line image having a first amount of intensity non-uniformity in a long-axis direction. The method also includes forming a secondary image that at least partially overlaps the line image. The method also includes scanning the secondary image in the long axis direction over at least a portion of the primary image while measuring an emissivity and a reflectivity from a portion of the wafer surface associated with the scanned secondary image. The method further includes calculating a wafer surface temperature based on the measured reflectivity and emissivity. The method additionally includes adjusting at least one of a scan speed and an intensity of the secondary image based on the calculated wafer surface temperature.

Additional features and advantages of the disclosure are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description present embodiments of the disclosure, and are intended to provide an overview or framework for understanding the nature and character of the disclosure as it is claimed. The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure, and together with the description serve to explain the principles and operations of the disclosure. The claims are incorporated into and constitute part of the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a general schematic diagram of an example line-image-forming optical system according to the present disclosure that uses primary and secondary laser systems;

FIG. 1B is similar to FIG. 1A and illustrates an example embodiment where the secondary laser beam is formed by diverting a portion of the primary laser beam;

FIG. 2 is a schematic view of an idealized line image;

FIG. 3A and FIG. 3B are idealized normalized intensity vs. distance (mm) plots for the idealized line image of FIG. 2 taken along the short-axis (scan) direction and the long-axis (cross-scan) direction, respectively, with the square (solid line) and Gaussian (dashed line) representing examples of idealized intensity profiles;

FIG. 4 is similar to FIG. 2 but shows a secondary image that is smaller than the line image (primary image) and that overlaps the line image, wherein the secondary image is scanned along the long-axis direction of the primary image according to a scan profile;

FIG. 5 is a prior art intensity contour plot obtained by measuring the thermal emission of a substrate irradiated with an example line image formed by a conventional line-image-forming system;

FIG. 6A and FIG. 6B plot the intensity vs. distance in the short-axis direction and long-axis directions, respectively, for the prior art intensity contour plot of FIG. 5;

FIG. 7A is an intensity contour plot of an example line image having a relatively high degree of intensity non-uniformity along its length, and also showing an example secondary image being scanned along the line image long axis;

FIG. 7B is an intensity vs. distance plot in the long-axis direction of the line image of FIG. 7A, and shows the secondary image being scanned along the line image long axis and the variation in secondary image intensity (dashed line) according to the scan profile;

FIG. 8A is an intensity vs. distance plot in the long-axis direction of the line image that shows the resultant time-averaged modified line image intensity profile formed by scanning the secondary image according to a scan profile along the long-axis direction;

FIG. 8B is an intensity contour plot corresponding to the modified line image of FIG. 8A, showing a greater degree of intensity uniformity along the long-axis direction for the time-averaged modified line image as compared to the intensity contour plot of FIG. 7A for the conventionally formed line image;

FIG. 9 is a plot of the intensity (counts) vs. distance (mm) in the long-axis direction, illustrating an example tilt in the intensity profile along the line image long axis;

FIGS. 10A and 10B are plots similar to FIG. 9 for a measured thermal emission image for a line image formed on a patterned silicon wafer and showing the resultant high-spatial-frequency features caused by light scattering from the wafer pattern;

FIG. 10C is similar to FIG. 10A and FIG. 10B and illustrates where the emission image has been filtered with a low-pass filter to remove the high-frequency modulation caused by light scattering by the wafer pattern;

FIG. 11 is a schematic diagram of an example laser annealing system that includes the line-image-forming system of the present disclosure for forming a scanned modified line image with relatively high intensity uniformity for thermal annealing a semiconductor wafer;

FIG. 12 is a plan view of a section of a semiconductor wafer illustrating an example wafer scan path for the scanning modified line image over the wafer surface, with the wafer scan path having adjacent linear scan path sections that are separated by a stepping distance DS that results in some overlap of the edges of the modified line images for adjacent linear scan path sections;

FIG. 13A is a schematic diagram of the line image intensity and corresponding line images for conventional line images (solid and dashed lines) associated with adjacent scan path sections of a wafer scan path when performing laser annealing, illustrating how an intensity gap can form when the line images are not substantially uniform along the long axis and when the adjacent scan path sections provide no line-image overlapped;

FIG. 13B is similar to FIG. 13A and shows a 50% line image overlap in the long-axis direction for adjacent scan path sections for a conventionally formed line image;

FIG. 13C is similar to FIG. 13B but shows the intensity profiles and line images for a modified line image for adjacent scan path sections of the wafer scan path, wherein the amount of overlap needed is much less than for the conventionally formed line image of FIG. 13B;

FIG. 14 is a schematic diagram of the line-image-forming optical system similar to that shown in FIG. 1A but having an enhanced configuration for the thermal emission detection system;

FIG. 15 is a close-up schematic view of an example collection optical system used to measure the amount of light from the secondary light beam that is reflected from the wafer surface as part of the process for calculating the local wafer surface temperature in real time; and

FIG. 16 is similar to FIG. 14 and illustrates an example embodiment that includes a pre-heat light source for pre-heating the wafer in carrying out the laser annealing process of the disclosure.

DETAILED DESCRIPTION

Reference is now made in detail to embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts.

It is noted that the term “line image” is used herein in to generally denote an elongate intensity distribution of light formed by a light beam at an image plane, and thus does not necessarily require an associated “object” in the classical sense. For example, the line image can be formed using beam-conditioning optics that cause the aforementioned light beam to come to a line focus at the image plane.

Also, a “time-averaged line image” is defined herein as a line image whose intensity is measured over a period of time and is averaged over that period of time.

Line-Image-Forming Optical System

FIG. 1A is a general schematic diagram of an example line-image-forming optical system (“system”) 10 according to the present disclosure. A Cartesian coordinate system is shown for the sake of reference. System 10 includes a primary laser system 20 that emits an initial primary laser beam 22 along an optical axis A1 that runs in the Z-direction. A beam conditioning optical system 30 is arranged along optical axis A1 downstream of primary laser system 20. Beam conditioning optical system 30 is configured to received initial primary laser beam 22 and form therefrom a line-image-forming beam 32 (also referred to hereinafter as the “primary light beam”) that forms a line image 36 (also referred to as the “primary image”) at an image plane IP that lies in an X-Y plane. Primary laser system 20 and beam-conditioning optical system 30 constitute an example of a primary optical system 21 having an optical path OP1.

FIG. 2 is a schematic view of an idealized line image 36 as formed at image plane IP. Idealized line image 36 has a “short axis” width W_(1X) and a “long axis” length L_(1Y). Idealized line image 36 of FIG. 2 also depicts (i.e., is representative of) an ideal “flat top” intensity contour plot for line image 36.

FIG. 3A and FIG. 3B are idealized normalized intensity vs. distance (mm) plots for an ideal line image 36 having a width W_(1X)˜0.025 mm and a length L_(1Y)˜10 mm. FIG. 2 can be thought of as an idealized intensity contour plot that has a single, sharply defined intensity contour at a normalized intensity of 1.

With reference again to FIG. 3A, note that a smooth profile with a single maximum along the short axis (e.g., a Gaussian or near-Gaussian) as shown by the dashed line curve is also a suitable short-axis intensity profile for ideal image 36.

With reference again to FIG. 1A, beam conditioning optical system 30 can include lenses, mirrors, apertures, filters, active optical elements (e.g., variable attenuators, etc.) and combinations thereof. Example beam conditioning optical systems 30 are disclosed in U.S. Pat. Nos. 7,514,305, 7,494,942, 7,399,945 and 6,366,308, and U.S. patent application Ser. No. 12/800,203, all of which are incorporated by reference herein.

In an example, a planar workpiece 40 having a surface 44 is arranged at image plane IP so that that the workpiece surface lies substantially in the image plane. In an example, workpiece 40 comprises a semiconductor wafer. In the discussion below, workpiece 40 is also referred to as wafer 40 depending on the context of the discussion.

The description of system 10 up until now describes a conventional line-image-forming optical system. However, with continuing reference to FIG. 1A, system 10 according to the present disclosure also includes a secondary laser system 50 that emits an initial secondary laser beam 52 along an optical axis A2 that also runs in the Z-direction and thus parallel to axis A1. System 10 also includes a scanning optical system 60 arranged along optical axis A2 and downstream of secondary laser system 50. In an example embodiment, a variable attenuator 56 is arranged between secondary laser system 50 and scanning optical system 60. Secondary laser system 50 and scanning optical system 60 define a secondary optical system 51 that has a secondary optical path OP2.

System 10 further includes a controller 70 operably connected to secondary laser system 50, to optional variable attenuator 56, and to scanning optical system 60 and is configured to coordinate the operation of these systems (and optional attenuator 56) as part of system 10 via electrical control signals S50, S56 (optionally), and S60, as described below.

Scanning optical system 60 is configured to receive initial secondary laser beam 52 and form therefrom a scanning laser beam 62 (also referred to as a “secondary light beam”) that forms a secondary image 66 at image plane IP. With reference to FIG. 4, scanning optical system 60 is configured to scan secondary image 66 based on a scan profile over at least a portion of line image 36. The scan profile scans the primary image in the direction of the long axis (i.e., the Y-direction), as illustrated by Y-direction arrows 68. Secondary image 66 is generally smaller than primary image 36 (i.e., has a smaller area) and at least partially overlaps primary image 36 when the secondary image is stationary.

In the example illustrated in FIG. 4, secondary image 66 completely overlaps line image 36, i.e., the secondary image falls within the line image: A “partially overlapping” secondary image would extend beyond a boundary of line image 36. Thus, the secondary image 66 is said to “at least partially overlap” line image 36, where in some cases this means that the secondary image completely overlaps the line image as shown in FIG. 4. Consequently, a “complete overlap” of the secondary and line images as this phrase is understood and used herein does not mean that the secondary image completely covers the line image. In some instances, secondary image 66 completely overlaps line image 36 and falls well within the line image 36, i.e., the secondary image has a width W_(2X) that is substantially smaller than the line image short-axis width W_(1X). In an example, the dimensions of line image 36 and secondary image 66 are defined by a select intensity value (i.e., an intensity threshold).

The purpose of scanning secondary image 66 relative to primary image 36 based on a scan profile is described in greater detail below. Secondary image 66 can be any one of a variety of general shapes such as a line, round, oval, rectangular, square, etc. that will accomplish the function of forming a modified line image 36′, as described below.

FIG. 1B is similar to FIG. 1A and illustrates an example embodiment of system 10 where initial secondary laser beam 52 is formed by diverting a portion of initial primary laser beam 22. In an example, this is accomplished by disposing a beamsplitter BS along axis A1 to divert a portion 22′ of the initial primary laser beam 22. A fold mirror FM is optionally used to direct laser beam portion 22′ along axis A2 so that this laser beam portion can serve as the initial secondary laser beam 52. Beam splitter BS and fold mirror FM constitute one example of a beam-splitting optical system 74 that directs a portion 22′ of initial primary laser beam 22 to form initial secondary laser beam 52. Other variations of beam-splitting optical system 74 are contemplated herein, including where beam splitter BS is replaced by a small mirror (not shown) that deflects a small piece of initial primary laser beam 22 to fold mirror FM. In the embodiment of FIG. 1B, system 10 still includes two optical paths OP1 and OP2, and secondary optical system 51 includes primary laser system 20 rather than secondary laser system 50.

In an example, laser beam portion 22′ is processed by an optional beam conditioning optical system 30′ configured to form a conditioned secondary light beam 52, i.e., one having a more uniform intensity over its cross-section than laser beam portion 22′. Beam conditioning optical system 30′ is similar to beam conditioning optical system 30 in that it can include lenses, mirrors, apertures, filters, active optical elements (e.g., variable attenuators, etc.) and combinations thereof, to form a suitable light beam 52 for use by scanning optical system 60 to form a suitable secondary light beam 62. In an example, controller 70 is electrically connected to beam conditioning optical system 30′ to control any active optical components therein via a control signal S30′.

Line Image Intensity Uniformity

FIG. 5 is a prior art intensity contour plot obtained by measuring the thermal emission of a semiconductor wafer irradiated with an example line image 36 formed by a conventional line-image-forming system. The intensity contours are based on a normalized intensity. The short-axis direction is expanded to emphasize the short-axis intensity variations.

FIG. 6A and FIG. 6B plot the intensity vs. distance in the short-axis direction and long-axis directions, respectively, for the intensity contour plot of line image 36 of FIG. 5.

With reference to FIG. 5 and FIGS. 6A and 6B, the long-axis intensity profile shows about a 20% variation in intensity in the range indicated by the parallel dashed lines of FIG. 6B. The line image intensity profile in the long-axis direction includes intensity variations that can be attributed to a number of different factors, such as diffraction, optical aberrations, optical misalignment or a combination thereof. Dynamic aberrations and/or misalignments typically give rise to intensity non-uniformities in the form of a time-varying tilt along the long axis. This phenomenon is sometimes referred to as “beam wobble.” The intensity profile of line image 36 can also have a static tilt due to for example residual heating effects or a static misalignment.

Substantial long-axis intensity non-uniformities (e.g., 20%) are not acceptable for certain applications such as laser annealing, wherein a high degree of temperature uniformity on a wafer is required during the annealing process.

FIG. 7A is an intensity contour plot of an example line image 36 having a relatively high degree of intensity non-uniformity along its long axis, and also shows an example secondary image 66 being scanned along the line image long axis. FIG. 7B is an intensity vs. distance plot in the long-axis direction of the line image of FIG. 7A, and shows the secondary image being scanned along the line image long axis as indicated by arrows 68, and the variation in secondary image intensity (dashed line 69) along the scan profile.

System 10 is configured for improving the intensity uniformity of line image 36 formed by just primary laser system 20 and beam conditioning optical system 30 by supplying additional (secondary) intensity via secondary image 66 where the line-image intensity is low. This in effect fills in the otherwise non-uniform intensity profile of line image 36 to form a modified line image 36′ that is a time-averaged combination of primary image 36 and a selectively scanned secondary image 66.

FIG. 8A is an intensity vs. distance plot in the long-axis direction of the modified line image 36′ and shows the resultant time-averaged modified line-image intensity profile formed by scanning the secondary image 66 according to a scan profile along the long-axis direction. The resultant modified line image 36′ has a time-averaged intensity profile closer to the ideal “flat top” intensity profile shown in FIGS. 2, 3A and 3B.

FIG. 8B is an intensity contour plot corresponding to the modified line image 36′ of FIG. 8A. The intensity contour plot of FIG. 8B has a greater degree of intensity uniformity in the long-axis direction as compared to the intensity contour plot of FIG. 7A.

Modified line image 36′ is time-averaged to achieve the required level of intensity uniformity, e.g., +/−5% or better. The time-averaging can be taken over a single scan pass or multiple scan passes of secondary image 66 over at least a portion of primary image 36, or over a single scan pass of the length of the primary image, or over multiple passes in the same direction, or multiple passes back and forth (i.e., in opposite directions) over the primary image.

Where system 10 is used for laser annealing, primary laser system 20 can include a high-power CO₂ laser and secondary laser system 50 can include a low-power CO₂ laser. Or, as discussed above in connection with FIG. 1B, a single high-power CO₂ laser can be used to form both the primary and secondary light beams 32 and 62.

In one example, secondary image 66 is scanned along the long axis of primary image 36 via the operation of scanning optical system 60, with the scanning time t_(s) of the scanned secondary image 66 being about the same as or shorter than the dwell time t_(d) of primary image 36. Here, the scanning time t_(s) is the time it takes to scan secondary image 66 over the scan path, and the dwell time t_(d) is the amount of time that line image 36 remains at a given location (point) at image plane IP or at a point on workpiece surface 44 when a workpiece 40 is arranged in the image plane.

In the case where line image 36 is scanned relative to a fixed image plane IP or relative to a workpiece arranged in the image plane (e.g., the workpiece is moved relative to the line image), then the dwell time t_(d) is the amount of time the line image covers a given point in the image plane or on the workpiece.

Using the above systems and methods, modified line-image 36′ can have an amount of time-averaged intensity non-uniformity that is less than the amount of intensity non-uniformity in line image 36. This can be accomplished by: i) maintaining secondary image 66 and at a substantially constant power and adjusting (i.e., speeding up and/or slowing down) the secondary-image scanning, ii) selectively changing the power of secondary image and scanning it at a substantially constant velocity, or ii) by a combination of methods i) and ii).

Where the amount of intensity of secondary image 66 needs to be varied in a select manner, adjustable attenuator 56 can be used and controlled by controller 70 via a control signal S56. Alternatively, or in combination therewith, controller 70 can modulate secondary laser system 50 using control signal S50

In an example, the scan profile can be configured so that secondary image 66 is scanned only over select portions of line image 36, i.e., only those portions where additional intensity is needed. This can be accomplished by the secondary image 66 having substantially “zero intensity” during select portions of the scan profile where no extra intensity needs to be added to the primary image.

Power Requirements for Laser Annealing

Typical intensity variations in a conventional line image for laser annealing applications are usually between 10% to 20% (i.e., +/−5% to +/−10%). Process temperature variations due to variations in the power density in the line image are accounted for through a power density variation parameter μ(y), where y is the long-axis dimension of line image 36. A typical value for μ(y) at an annealing temperature of about 1,300° C. is about 1% to 2%. Typical dimensions for primary image 36 are length L_(1Y)=10 mm and width W_(1X)=0.1 mm, while a typical power P of primary laser beam 22 is 500 W. Thus, an example power density or intensity I₁ (power P₁ per area A₁) associated with primary image 36 is: I ₁ =P ₁ /A ₁ =P/(L _(1Y) ·W _(1X))=(500 W)/([10 mm]·[0.1 mm])=500 W/mm².

The energy density is given by E=I₁·t_(d), where t_(d) is the dwell time for the primary light beam. The variation of energy density is then ΔE=μ·I ₁ ·t _(d).

Secondary laser system 50 needs to provide the secondary image 66 with an energy density E sufficient to compensate for the variation of energy density ΔE in primary image 36.

In an example, the width W_(2X) of secondary image 66 is substantially the same as the width W_(1X) of primary image 36, i.e., W_(1X)˜W_(2X). In this example, secondary image 66 has an area A₂=W_(2X)·W_(2Y)=W_(1X)·W_(2Y). Also in an example, secondary image 66 is scanned over primary image 36 in a scan time t_(s) that is a fraction of the primary light beam dwell time t_(d), so that t_(s)=v·t_(d), where 0<v<1.

The power P₂ that needs to be provided by secondary laser system 50 is estimated from the equation: Max{(μ)}·I ₁ ·t _(d)=(P ₂ ·t _(s))/(W _(1X) ·W _(2Y)) Rearranging and using I₁=P₁/(W_(1X)·L_(1Y)), P2 can be expressed as: P ₂ =P ₁ {W _(2Y) /L _(1Y)}·{(Max(μ))/v}

Using P₁=500 W, max (μ)=0.02, v=0.1, W_(2Y)=0.1 mm, L_(1Y)=10 mm, the secondary power P₂˜1 W. Using a comfortable safety margin of 10× yields P₂˜10 W. This amount of secondary power is readily provided by a number of commercially available CO₂ lasers, and can also be obtained by re-directing a portion of a high-power CO₂ light beam.

Secondary Image Scanning and Control

For constant velocity scanning of secondary image 66 over at least a portion of primary image 36 at a scanning velocity V in the long-axis direction (i.e., Y-direction), the power in the secondary image 66 as a function of y is given by: v·P ₂(y)=P ₁·μ(y)·[W _(1X) /W _(1Y)] where the y position of the center (e.g., centroid) of secondary image 66 at a given scanning time t_(s) is given by y=V·t_(s).

For modulation of the dwell time t_(d), v goes from being a constant to being a function of the distance along the primary image, i.e., v→v(y).

As described above, static and dynamic line-image non-uniformities can arise. The static non-uniformities can be caused by beam modulation, residual heat, etc, while the dynamic non-uniformities (so-called beam wobble) can be caused by variations of the refractive index in a beam path and vibrations of the optics. Frequencies of the dynamic variations in line-image uniformity typically do not exceed 100 Hz.

One method of compensating for static non-uniformities in primary image 36 includes scanning the primary image over a set of one or more test (blanket) wafers. The method also includes measuring the thermal emission (emissivity) from each wafer to obtain a measure of the variation in intensity of the primary image 36 in the long-axis direction. Here, it is assumed that the emission from the wafer is proportional to the intensity of primary image 36. More precisely, the wafer temperature is proportional to the intensity, with the emission from the heated wafer being related to the temperature through Planck's equation. This assumption is generally appropriate for the intensity measurements contemplated herein.

Statistical analysis (e.g., averaging) of the wafer measurements can be used to determine a representative primary image 36R (i.e., representative intensity profile) for primary image 36, which in turn can be used to define the scan profile for secondary image 66 that substantially compensates for the static non-uniformities in representative primary image 36R. The resultant representative primary image 36R can be stored in memory (e.g., in controller 70) and can be used for temperature-based close loop control used when scanning product wafers.

To account for any drifts in the operating parameters of system 10, such as aging of the laser in primary laser system 10 and the components in beam conditioning optical system 30, the representative primary image 36R can be periodically updated, e.g., by performing more wafer exposures and measurements using primary image 36. The representative primary image 36R can also be updated periodically as needed in view of certain events, such as after major maintenance procedures involving system 10, such as optical re-alignments, replacement of optical components, servicing or replacing lasers, etc.

To compensate for time-varying intensity non-uniformities in line image 36, the scan profile for secondary image 66 can be controlled using a real-time feedback system. With reference again to FIG. 1A, in an example, system 10 includes a thermal emission detection system 80 (e.g., a CMOS imaging camera or a CCD array) capable of detecting thermal emissions at temperatures of about 1,300° C. to capture an emission image (emission profile) along the long axis. Thermal emission detection system 80 is arranged to view primary image 36 and capture an emission image thereof and generate an electrical signal S80 representative of the captured emission image. Electrical signal S80 is provided to controller 70, which in an example embodiment is configured to store and process the emission images embodied in electrical signal S80. In one example, thermal emission detection system 80 images at a rate of 200 frames per second or greater to provide a sufficient sampling frequency for intensity variations that occur in primary image 36.

Controller 70 processes electrical signals S80 and performs a beam profile analysis (e.g., statistical averaging of emission images and conversion of measured emission to intensity) to form a representative primary image 36R. The real-time compensation of the representative primary image 36R is then accomplished by calculating a secondary image scan profile based on the representative primary image 36R. Controller 70 then provides control signal S50 to secondary laser system and control signal S60 to scanning optical system 60 to carry out the secondary image scanning according to the calculated secondary image scan profile.

In an example embodiment, Controller 70 is or includes a computer such as a personal computer or workstation, or a stand-alone control system utilizing any combination of programmable logic devices such as any one of a number of types of micro-processors, central processing units (CPUs), floating point gate arrays (FPGAs) or application specific integrated circuits (ASICs). In addition to one or more such programmable logic devices, controller 70 also may include a bus architecture to connect the processor to a memory device, such as a hard disk drive, and suitable input and output devices (e.g., a keyboard and a display, respectively).

In an example, the FPGA can be configured to perform emission image analysis and a real-time controller unit with shared memory and direct memory access (DMA) data transfer to the shared memory.

In a modification to this embodiment, controller 70 may utilize distributed logic, with an image acquisition and processing sub-system, containing image acquisition hardware and utilizing a programmable logic device (e.g., an FPGA) for control and processing of the thermal image data. In an example, this sub-system communicates with a real time control sub-system, which could utilize a micro-processor and related peripherals, running a real-time operating system. The real time control sub-system can be utilized to communicate between other system controllers, as well as perform command and control functions related to the image processing and control of the secondary image. Communication between sub-systems could be through any combination of a communications interface (e.g., Ethernet, RS422), a shared logic bus and a shared memory bus.

Dynamic instabilities in the intensity of primary image 36 often take the form of a linear intensity tilt, as shown in the FIG. 9, which plots the intensity (counts) vs. long-axis distance (mm). The tilted nature of the intensity profile is shown via dotted line 89. The tilt of the intensity profile typically changes with time, usually with a frequency of about 100 Hz or less. The random nature of certain types of dynamic intensity variations such as the aforementioned tilting precludes measuring them beforehand and then trying to compensate using secondary image 66.

Emission images can be very complex when measured on patterned wafers. FIGS. 10A and 10B are representative plots similar to FIG. 9 for the measured emission image of line image 36 formed on a patterned silicon wafer. Each plot includes a region 90 where the emission intensity is modulated at a relatively high spatial frequency due to the wafer pattern formed by various device structures (lines, shapes, vias, kerfs, alignment marks, etc.) formed when processing the wafer to form semiconductor chips (i.e., integrated circuits).

Thus, in an example, the emission images from thermal emission detection system 80 as embodied in signals S80 are low-pass filtered and then processed in a manner that allows for comparison to the static representative primary image 36R. The appropriate adjustment is then made to the scanning profile for secondary image 66. FIG. 10C is similar to FIG. 10B, but where the emission image (signal S80) has been filtered with a low-pass filter to remove the high-frequency modulation caused by the wafer pattern.

For changes to primary image 36 that occur at a given frequency f (e.g., 100 Hz), the emission image acquisition and subsequent secondary image scan profile calculation need to occur with a frequency of about 2 f (e.g., 200 Hz).

Laser Annealing System

Laser annealing in semiconductor processing is typically performed on patterned wafers. The absorption on patterned wafers varies with the pattern dimensions, the pattern density and the laser wavelength. It has been shown that laser annealing with a wavelength much longer than the pattern dimensions reduces scattering and thus increases wafer absorption.

FIG. 11 is a schematic diagram of a laser annealing system 100 that includes the line-image-forming system 10 of the present disclosure. An example laser annealing system 100 for which the line-image-forming optical system 10 is suitable for use is described in, for example, U.S. Pat. Nos. 7,612,372, 7,154,066 and 6,747,245, which patents are incorporated by reference herein.

System 10 is shown generating primary light beam 32 and scanned secondary light beam 62 to form modified line image 36′. Primary and secondary light beams 32 and 62 have a wavelength (e.g., nominally 10.6 microns from either the same or from respective CO₂ lasers) that is/are capable of heating wafer 40 under select conditions. Such conditions include, for example, heating wafer 40, or irradiating the wafer with radiation from a pre-heat light source (not shown), with the radiation having a bandgap energy greater than the semiconductor bandgap energy of the wafer, thereby causing the wafer to absorb primary and secondary light beams 32 and 62 to a degree sufficient to heat the wafer to annealing temperatures.

An example of irradiating the wafer with a third (pre-heat) light source to make the wafer more absorbent at CO₂ laser wavelengths is described below in connection with FIG. 16, and is also described in U.S. Pat. Nos. 7,098,155, 7,148,159 and 7,482,254, all of which are incorporated by reference herein. In a preferred embodiment, primary and secondary light beams 32 and 61 have the same or substantially the same wavelength.

Wafer 40 is supported by a chuck 110 having an upper surface 112. In an example, chuck 110 is configured to heat wafer 40. Chuck 110 in turn is supported by a stage 120 that in turn is supported by a platen 130. In an example embodiment, chuck 110 is incorporated into stage 120. In another example embodiment, stage 120 is movable, including being translatable and rotatable.

Wafer 40 is shown by way of example as having semiconductor structures in the form of source and drain regions 150S and 150D formed at or near wafer surface 44 as part of a circuit (e.g., transistor) 156. Note that the relative size of the source and drain regions 150S and 150D in circuit 156 compared to the dimensions of wafer 40 is greatly exaggerated in FIG. 11 for ease of illustration. In practice, source and drain regions 150S and 150D are very shallow, having a depth into the wafer surface 44 of about one micron or less. Source and drain regions 150S and 150D constitute the above-mentioned wafer patterning that can cause high-frequency modulation when capturing an emission image of primary image 36.

In an example embodiment, system 100 further includes a controller 170 electrically connected to system 10 (including controller 70 therein; see FIGS. 1A and 1B) and to a stage controller 122. Stage controller 122 is electrically coupled to stage 120 and is configured to control the movement of the stage via instructions from controller 170. Controller 170 is configured to control the operation of system 100 generally, and in particular system 10 and stage controller 122.

In an example embodiment, controller 170 is or includes a computer, such as a personal computer or workstation, available from any one of a number of well-known computer companies such as Dell Computer, Inc., of Austin Tex. Controller 170 preferably includes any of a number of commercially available micro-processors, a suitable bus architecture to connect the processor to a memory device, such as a hard disk drive, and suitable input and output devices (e.g., a keyboard and a display, respectively).

With continuing reference to FIG. 11 and also to FIGS. 1A and 1B, primary light beam 32 is directed onto wafer surface 44 to form primary image 36 thereon, while secondary light beam 62 is scanned according to a scan profile as discussed above to scan secondary image 66 over at least a portion of the primary image to form modified line image 36′.

In an example embodiment, modified line image 36′ is scanned over wafer surface 44, as indicated by arrow 180. FIG. 12 is a schematic diagram of an example wafer scan path 200 (dashed line) over which modified line image 36′ is scanned. Wafer scan path 200 includes a number n of linear scan path sections 202-1, 202-2, . . . 202-j . . . 202-n. Adjacent linear scan path sections (e.g., 202-j and 202-j+1) are formed by stepping modified line image 36′ by a stepping distance DS from one linear scan path to the next. Stepping distance DS is usually less than line-image length L_(1Y) so that there is at least some overlap between line images for adjacent scan path sections 202. The amount of this line-image overlap for conventional laser annealing systems versus those disclosed herein is discussed in greater detail below.

Scanning modified line image 36′ over wafer scan path 200 results in rapid heating of the wafer surface (down to a depth of about 1 micron or less) up to a temperature (e.g., between 1000° C. and 1,300° C.). This is sufficient to activate dopants in the source and drain regions 150S and 150D, while also allowing for rapid cooling of the wafer surface so that the dopants do not substantially diffuse, thereby maintaining the shallowness of the source and drain regions.

A typical scan velocity of modified line image 36′ over wafer surface 44 for linear wafer scan path sections 202 ranges from 25 mm/sec to 1000 mm/sec. In an example, one or both of modified line image 36′ and wafer 40 can move during scanning to define wafer scan path 200.

Throughput Enhancements

Laser annealing in semiconductor processing requires very precise temperature control over the entire region being annealed. Most often, the peak temperature drives the annealing process.

With reference to FIG. 13A, in the cases where the annealing beam forms a line image 36 that is non-uniform in the long-axis direction, and particularly at line image ends 36E, a gap G can form between adjacent scan sections 202 of wafer scan path 200, resulting in portions of the underlying wafer surface 44 (FIG. 11) not being fully exposed if the stepping distance DS is too great. A gap G will generally occur if the stepping distance DS=L_(1Y), i.e., the stepping distance is the same as the length of line image 36.

It is noted here that in an example, line image length L_(1Y) is defined by the distance in the long-axis direction over which laser annealing takes place when the line image is scanned over the wafer. This measure usually corresponds to a given intensity threshold in the line image and depends on the line-image scanning speed (or equivalently, the dwell time t_(d)).

Thus, it is generally necessary to overlap line images 36 for adjacent scan sections 202 of wafer scan path 200 to improve annealing uniformity over wafer 40. In a conventional laser annealing system, line image 36 is “stepped” by half its length L_(1Y) or less (i.e., DS≦L_(1Y)/2, or at least 50% overlap) between adjacent path sections 202 so that each point on the wafer is scanned twice by the line image. This is schematically illustrated in the FIG. 13B, which shows two overlapping long-axis line image profiles and line images for adjacent scans path sections 202 for a conventional line image 36 having substantial intensity non-uniformity at its ends 36E. Unfortunately, by having to substantially overlap the line images for adjacent scan path sections, wafer throughput is reduced.

By way of example, consider a 10 mm long line image 36 and a 5 mm stepping distance DS between adjacent scan path sections of the wafer scan path 200 (i.e., 50% line-image overlap). Laser annealing a 300 mm wafer requires (300 mm)/(5 mm)=60 steps. For a smaller stepping distance DS=2.5 mm (i.e., 75% line-image overlap), each point on the wafer is annealed four times and the wafer scan path 200 requires 120 steps.

With reference to FIG. 13C, modified line image 36′ can be formed to have much steeper intensity profiles at edges 36E′ so that that substantially less overlap of line images 36′ for adjacent scan path sections 202 is required. Throughput is thus improved by increasing the stepping distance DS between adjacent scan path sections to be much closer to the full length L_(1Y) of modified line image 36′.

In an example, the amount of overlap required between adjacent scan path sections 202 for modified line image 36′ is less than 50% and can be a small as 5% (i.e., L_(1Y)/20≦DS≦L_(1Y)/2). A typical line-image overlap range for modified line image 36′ is 5% to 10% (i.e., L_(1Y)/20≦DS≦L_(1Y)/10) Thus, for a 10 mm length for modified line image 36′, the stepping distance DS can be as large as 9.5 mm, resulting in only 32 steps for laser annealing a 300 mm wafer.

Wafer throughput in laser annealing is directly related to the number of steps between adjacent scan path sections in the wafer scan path 200. A typical “step and scan” for wafer scan path 200 takes about 1 second. Thus, for the above laser annealing examples, a conventional laser annealing system requires between about 60 seconds and 120 seconds to laser anneal a wafer for a line-image overlap between 50% and 75%. In contrast, the laser annealing system of the present disclosure takes between about 32 seconds and 34 seconds for a line-image overlap of between 5% and 10%. Thus, wafer throughput for laser annealing can be increased by close to 2× by performing the annealing process using modified line image 36′

Enhanced Thermal Emission Detection

As discussed above in connection with FIGS. 1A and 1B, system 10 includes a thermal emission detection system 80. During the time required for emission pattern capturing and processing, it is possible with the thermal emission detection system 80 as shown in FIGS. 1A and 1B that the thermal emission pattern may change. This could lead to a change in the emission image and in turn could lead to a less-than-optimal modified line image 36′.

FIG. 14 is similar to FIG. 1A and illustrates an example embodiment of system 10 that has an alternative configuration for thermal emission detection system 80. The thermal emission detection system 80 of FIG. 16 includes a dichroic mirror 82 disposed along axis A1 between the scanning optical system 60 and secondary laser system 50. Dichroic mirror 82 is configured to pass initial secondary laser beam 52 of wavelength λ₅₂ and reflect thermal emission light 63 from wafer surface 44, where the thermal emission light has a wavelength λ_(E) that is close to but not identical to wavelength λ₅₂. Dichroic mirror 82 is disposed along axis A1 in such a way that it defines an optical axis A3 that is an angle to axis A2, e.g., at a right angle thereto. Thermal emission detection system 80 also includes in order along optical axis A3, a polarizer 84, a focusing lens 86, a band-pass filter 88 and a photodetector 92. Photodetector 92 includes one or more photodetector elements 94. In an example, photodetector 92 includes a single photodetector element 94.

Thermal emission detection system 80 includes scanning optical system 60, which is also used to form from initial secondary laser beam 52 scanning laser beam 62 that scans over wafer surface 44. Thermal emission detection system 80 thus shares substantially the same field-of-view (FOV) associated with secondary image 66. This is schematically illustrated by thermal emission light 63 of wavelength λ_(E) emitted from wafer surface 44 and that is collected by scanning optical system 60. Thus, the FOVs of the scanning optical system 60 and the thermal emission detection system 80 (which also includes the scanning optical system) substantially overlap and track each other during scanning of secondary image 66.

In the operation of system 10 as modified per FIG. 14, emission light 63 is emitted by wafer surface 44 in response to being heated by modified line image 36′ (or just by secondary image 66). Emission light 63 is collected by scanning optical system 60 and is directed to dichroic mirror 82. Dichroic mirror is configured (e.g., with coatings, not shown) to reflect emission light 63 down axis A3 to polarizer 84 that has the same polarization as secondary laser system 50. The polarized emission light 63 proceeds to focusing lens 86, which focuses the emission light on to photodetector 92. Filter 88 disposed in front of photodetector 92 serves to filter out extraneous wavelengths outside of the narrow wavelength band Δλ_(E) associated with emission light 63 (wavelength λ_(E) can be considered a center wavelength of the narrow emission light wavelength band Δλ_(E)).

Thus emission light 63 is collected point by point while modified line image 36′ is scanning over wafer surface 44. In an example, the emission wavelength λ_(E) is close to wavelength λ₅₂ of secondary laser beam 52 to keep aberrations to within an acceptable tolerance. In an example focusing lens is configured to at least partially compensate for aberrations that arise from scanning optical system operating at emission wavelength λ_(E). In an example, λ_(E) differs from λ₅₂ by between 100 and 200 nm.

Thermal emission detection system 80 of FIG. 14 allows for the thermal emission light 63 from wafer surface 44 to be essentially simultaneously detected with the scanning of secondary image 66. Since the detection of emission light 63 accomplished using a fast photodetector 92, the corresponding electrical emission signal S80 is almost immediately available for closed-loop control of secondary image 66. This improves the speed at which secondary image 66 can be varied to compensate for intensity non-uniformities in line image 36. The configuration of system 10 of FIG. 14 is also less complex than that shown in FIGS. 1A and 1B in part because it does not require 2D image acquisition and image post-processing.

Temperature Measurement

To accurately control the temperature of wafer surface 44, one needs to be able to accurately measure its temperature. The detection of emission light 63 as described above by itself does not provide the wafer surface temperature. To measure the temperature of wafer surface 44, the emissivity £ must be measured. At a given temperature, the emissivity ε depends on the wavelength λE, viewing angle, and polarization of emission light 63.

One method of measuring emissivity ε is to determine reflectivity and transmission of the wafer at wavelength λ_(E). In an example, this is accomplished by employing secondary laser beam 62. If the wavelength λ₅₂ of secondary laser beam 52 is above or close to the Si absorption edge (i.e., about 1.1 μm), then the emissivity ε can be measured by measuring (or otherwise determining) the reflectivity and transmissivity of secondary laser beam incident upon wafer 40. However, where λ₅₂<1 μm or for λ₅₂>1 μm in combination with the high wafer surface temperatures associated with laser annealing, the wafer transmissivity can be neglected and only a measurement of wafer reflectivity is needed.

For accuracy, it is best if as much reflected light 62R from secondary light beam 62 that reflects from wafer surface 44 is collected. FIG. 15 is a close-up view of a collection optical system 300 arranged to collect reflected light 62R. Collection optical system 300 is shown arranged relative an scanning optical system 60, which in the example shown includes a scanning mirror 61M and a focusing lens 61L. Collection optical system 300 is incorporated into system 10 and includes along an axis A4 an integrating sphere 310 having an aperture 312. A photodetector 320 is arranged adjacent aperture 312 to detect light that exits the integrating sphere at the aperture.

In an example, at least one neutral density filter 316 is disposed between aperture 312 and photodetector 320 to control the intensity of light reaching photodetector 320. Photodetector 320 generates a photodetector signal S320 representative of the power in the reflected light 62R collected by integrating sphere 310 and provides this signal (hereinafter, the reflected-power signal) to controller 70.

With reference again to FIG. 14, system 100 includes a power sensor 350 configured to measure in real time the amount of power incident upon wafer surface 44. In an example, power sensor 350 is shown incorporated into secondary laser system 50. Power sensor 350 generates an electrical signal SP (hereinafter, the emitted-power signal) representative of the detected power, which in the example shown in FIG. 14 is representative of the power in initial secondary laser beam 52. Power sensor 350 provides an electrical signal SP to controller 70. Note that power sensor 350 can be located anywhere between secondary laser system and wafer surface 44.

In the case shown in FIG. 14 where the power sensor 350 is located upstream of the scanning optical system, the transmission of the scanning optical system needs to be accounted for in determining the amount of power in secondary light beam 62 that is actually incident wafer surface 44. In particular, the transmission of scanning optical system 60 can be provided to controller 70 to calculate the amount of power in secondary laser beam 62.

The emitted-power signal SP and the reflected-power signal S320 are measured in real time. By comparing the two signals (including any calculation regarding the transmission of scanning optical system 60 as described above), the emissivity ε is calculated on a point-by-point basis as secondary image 66 scans over wafer surface 44. The calculated emissivity ε is then employed to obtain a local temperature measurement that is insensitive to emissivity variations due to any pattern present on wafer surface 44. This in turn allows for closed-loop control of the amount of power required in secondary laser beam 62 to form modified line image 36′.

In an example, the temperature T is determined from the measured emissivity ε by solving the following equation:

$W_{\lambda} = {ɛ \cdot \Omega \cdot \frac{C_{1}}{\lambda_{E}^{5}\left( {{\mathbb{e}}^{{C_{2}/\lambda_{E}}T} - 1} \right)}}$ where ε is measured emissivity, Ω is a solid angle over which the thermal emission light 63 is collected, C₁ and C₂ are known constants, and W_(λ) is a measured emission signal. Thermal emission detection system 80 is preferably calibrated at a known temperature, such as the melt temperature associated with the annealing process.

Since the emissivity ε is a function of wavelength λ_(E), making λ_(E) sufficiently close to λ₅₂ (e.g., 150 nm) guarantees a sufficiently accurate temperature calculation, and therefore an accurate closed-loop control of secondary laser system 50.

Reflected light 62R includes both specular and non-specular components, the latter arising mainly due to wafer surface patterning. Consequently, the accuracy of the emissivity measurement is a function of the numerical aperture of collection optical system 300. In an example, collection optical system 300 has a numerical aperture as high as 0.2. In an example, collection optical system 300 is configured with a numerical aperture such that any errors in the emissivity calculation arising from not collecting all of the reflected light 62R are insubstantial when compared to a wafer surface temperature measurement that does not compensate for emissivity variations.

FIG. 16 is similar to FIG. 14 and illustrates an example embodiment of system 10 that includes a pre-heat light source 400 arranged along an optical axis A5 that intersects optical axis A2. Pre-heat light source 400 emits a pre-heat light beam 402 along optical axis A5. Pre-heat light beam 402 has a wavelength λ_(PH)<1 μm. Example pre-heat light sources include diode lasers, 532 nm solid state lasers, green fiber lasers, and the like.

A dichroic mirror 410 is disposed at the intersection of optical axes A2 and A5, which is adjacent secondary laser system 50. Dichroic mirror 410 and is configured to reflect pre-heat light beam 402 and transmit initial secondary laser beam 52. Pre-heat light beam 402 is then provided to wafer surface 44 along substantially the same optical path as initial secondary laser beam 52 and secondary laser beam 62 to form an image 460 at wafer surface 44. Dichroic mirror 410 thus allows the pre-heat light beam 402 and initial secondary laser beam 52 to simultaneously travel through scanning optical system 60.

Dichroic mirror 82 of thermal emission detection system 80 is configured to transmit pre-heat light beam 402. Also in an example embodiment, scanning optical system 60 is configured to accommodate the difference in wavelength between initial secondary light beam 52 and pre-heat light beam 402. Also in an example embodiment, an optical system 420 is arranged between pre-heat light source 400 and dichroic mirror 410. Optical system 420 is configured to work in conjunction with scanning optical system 60 to form an image 460 at wafer surface 44.

The configuration of FIG. 16 may be used, for example, where the temperature of wafer 40 needs to be raised prior to performing laser annealing. An example where such pre-heating is advantageous is described in U.S. Patent Application Publication no. 2010/0084744, which is incorporated by reference herein.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents. 

1. A line-image-forming optical system for thermally annealing a semiconductor wafer having a surface, comprising: a primary optical system configured to form from a primary laser beam a line image having a long axis and a first amount of intensity non-uniformity along the long axis; a secondary laser system that generates a secondary laser beam having a secondary laser beam wavelength and a secondary laser beam intensity; a scanning optical system having a first field of view configured to receive only the secondary laser beam and form therefrom a secondary image at the wafer surface, wherein the secondary image at least partially overlaps the line image and is scanned over at least a portion of the line image to form a time-averaged modified line image having a second amount of intensity non-uniformity that is less than the first amount; a thermal emission detection system configured to detect thermal emission light from the wafer surface through the scanning optical system and over a second field of view substantially the same as the first field of view of the scanning optical system, the thermal emission detection system configured to generate an electrical signal corresponding to the detected thermal emission light; and a controller configured to receive the electrical signal and in response thereto adjust at least one of the secondary laser beam intensity and a scanning speed of the secondary laser beam.
 2. The system of claim 1, wherein the thermal emission light has an emission wavelength that differs from the secondary laser beam wavelength by between 100 nm and 200 nm.
 3. The system of claim 1, wherein the thermal emission detection system further comprises: a dichroic mirror disposed in an optical path between the secondary laser system and the scanning optical system, the dichroic mirror configured to transmit the secondary laser beam and reflect the thermal emission light; a polarizer arranged to receive and polarize the reflected thermal emission to have a same polarization as the secondary laser beam; a focusing lens arranged focus the polarized thermal emission light; a photodetector arranged to receive the focused polarized thermal emission light; and a band-pass filter arranged between the dichroic mirror and the photodetector and configured to pass the thermal emission light and block light having the secondary laser beam wavelength.
 4. The system of claim 1, further comprising: a collection optical system configured to receive light from the secondary laser beam that reflects from the wafer surface and in response thereto generate a photodetector signal representative of the amount of power in the reflected light; a power sensor in the secondary laser system configured to measure an amount of power in the second laser beam; and the controller configured to calculate a reflectivity of the second laser beam based on the amount of power in the reflected light and the amount of power in the second laser beam.
 5. The system of claim 4, further comprising the controller configured to calculate a temperature of the wafer surface based on the measured thermal emission and the calculated reflectivity.
 6. The system of claim 4, wherein the collection optical system further comprises an integrating sphere and a photodetector.
 7. The system of claim 1, further comprising: a pre-heat light source that emits a pre-heat light beam having a wavelength less than 1 micron, the pre-heat light beam being directed to the wafer surface via the scanning optical system.
 8. The system of claim 1, further comprising a primary laser system that forms the primary laser beam.
 9. The system of claim 8, wherein the primary laser system includes a CO₂ laser.
 10. The system of claim 1, wherein the secondary image falls within the line image. 